CO2 sensor ADS and NL are based on principle of NDIR1) method. So the sensor drift is the normal and unavoidable aging mainly of the infrared light source within the sensor. This natural drift can be up or down. Self-calibration feature is designed to correct all sensor drifts including aging of the light source. The sensor drift is about a few ppm/ month.
Outside levels of CO2 are generally very low at around 400 ppm. Inside buildings people are the major source of CO2. When a building is unoccupied for a few hours, four or more CO2 levels will tend to drop to outside background levels. This is especially the case if the building operational schedule includes a pre-occupancy purge of fresh air into the building prior to the start of the day.
Auto-calibration which stands for “Automatic Background Calibration” utilizes the computing power in the sensor’s on-board microprocessor to remember the lowest CO2 concentration that takes place every 24 hours. The sensor assumes this low point is at outside levels. The sensor is also smart enough to discount periodic elevated readings that might occur if for example a space was used 24 hours per day over a few days even weeks. Once the sensor has collected three weeks worth of low concentration points it performs a statistical analysis to see if there has been any small changes in the sensor reading over background levels that could be attributable to sensor drift. If the analysis concludes there is drift, a small correction factor is made to the sensor calibration to adjust for this change. The figure below shows CO2 concentrations as they might occur in time in an office space with peak concentration occurring in the morning and afternoon of each day. If a statistically relevant change in the data shows a shift above or below background, a slight adjustment is made to sensor calibration as shown by the solid level line. Every day the sensor looks at the past three weeks data and determines if a calibration adjustment is necessary. A similar type of approach of nightly calibrations is often used for other HVAC equipment such as VAV boxes.